Modulation control



June 25, 1946. CHARCHlAN 2,402,598

MODULATION CONTROL Filed Aug. 19, 1942 AUDIO AMPLIFIER INPU T 20 STAGE+L' +H +L '+H INVENTOR BENJAMIN MALCOLM CHARCH/AN ATTORNEY Patented June25, 1946 PATENT OFFICE MODULATION coN'raoL Benjamin Malcolm Charchian,New York, N. Y.,

assignor to Federal Telephone and Radio Corporation, a corporation ofDelaware Application August 19, 1942, Serial N01 455,313

5 Claims. (01. 179-1715) 1 This invention relates to a modulationcontrol and more particularly to an arrangement for reducing the gain ofan audio frequency amplifier supplying an amplitude modulating potentialto a radio frequency carrier.

An object of this invention is to provide a novel arrangement forpreventing over-modulation of a radio frequency carrier.

Another object of this invention is to reduce the gain of an audiofrequency amplifier before over-modulation of a radio frequency carriertakes place. a

A further object of this invention i to anticipate over-modulation of'aradio frequency carrier by correcting the gain of an audio frequencyamplifier before such over-modulation occurs.

A still further-object of this invention is to prevent over-modulationof a radio frequency carrier by reducing the gain of anaudio frequencymodulating amplifier whenever th audio frequency voltage approaches thehigh potential applied to the plate of the modulated radio frequencyamplifying stage.

Other objects and advantages of the present invention will be apparentfrom a' study o the following description of a preferred embodimentthereof, illustrated in the accompanying draw-.

ing, in which the single figure illustrates a circuit diagram of apreferred form of gain reducer in accordance with the present invention.

As shown in the drawing the invention may be applied to a conventionalradio frequency amplifying stage, generally indicated at A, to the platecircuit of which a suitable audio input B is connected through aconventional audio amplifier C and a transformer I 0. The plate of theR.-F. amplifying stage A is supplied with a suitable high potential fromsource H.

In order to prevent over-modulation of the radio frequency carrier fromaudio input B and amplifier C, the arrangement in accordance with thepresent invention serves to reduce the gain of the amplifier C wheneverthe audio frequency modulating voltage approaches that of the highpotential source H. Such gain reduction is preferably applied to anearly stage of the audio frequency amplifier and, in the form of theinvention illustrated, the gain of the first audio frequency amplifiertube It is controlled. This tube I4 is preferably, although notnecessarily, of the pentode type having a control grid I6, indirectlyheated cathode i8, plate 20, screen grid 22, and suppressor grid 24.

Thecontrol arid It is connected to a suitable low potential source Lwhile the cathode II is connected to a suitably low potential source L,

the voltages of such sources being so chosen that under normal operatingconditions the tube It will provide full gain in its amplifying stage. 5The screen grid 22 may be connected to a suitable high potential sourceH', while the suppressor grid 24 is connected to the control grid itthrough a suitable resistorZB.

The plate 28 of a diode 30 i likewise connected to the control grid potetial source L, whereas the cathode 32 of this tu receives its power froma suitable resistor 34 serially connected between the modulatingtransformer i0 and the radio frequency amplifier A. The resistor 3! maybe shunted by a suitable capacitor 36 to permit a by-passing of thealternating currents. For purposes to be explained hereinafter, the gridit of the audio frequency tube It is preferably connected to the plate28 of the diode 30 through a suitable smoothing network generallyindicated at D, while a suitable high resistor 38 is interposed betweenthe potential source L on the one hand, and the plate 28 and grid It onthe other hand.

The system in accordance with the present invention will operate toreduce the gain in the audio frequency amplifier C to preventovermodulation of the radio frequency carrier in the following manner:Under normal operating con- 30 ditions when the audio frequency outputis such thatthere is no danger of over-modulation, the relativepotentials L and L on the cathode i8 and grid i6, respectively, of theamplifier tube It will be so chosen that the tube It will operate 5 withfull gain. As a practical example of the invention, the potential L maybe 53 volts and that of L may be 50 volts. If the potential source H isat a suitable high potential for operating the radio frequency amplifierA, say at 500 volts,

40 the diode 30 whose cathode 32 is connected to this high potentialsource will not operate, since the potential of its plate 28 will onlybe 50 volts. However, whenever the negative peak of the modulatingvoltage from the amplifier C epproaches the voltage of the highpotential source H to such a degree that the mean effective potential ofthe cathode 32 drops below the potential of the plate 28, or, in theexample given, be-

low volts, the diode 30 becomes conductive.

so This, in turn, will reduce the potential on the diode l and the gridl8 renders the action sufficiently gradual, and if the resistor II. issulficiently high in comparison with the internal im- Dedance of thediode Ill whenever the latter becomes conducting, the circuit will havean asymmetric time constant, being quick to reduce gain and slow torestore the gain to normal.

It will be noted that the cathode of the "diode 30 is heated by aportion of the plate current supplied to'the radio frequency amplifierA. This is necessary since the diode is operated with a its cathode at ahigh voltage with respect to ground and avoids the use of a filamenttransformer which would introduce an undesirable large capacity toground, shunting the secondary of the audio modulating transformer IO.

' It will be obvious to those skilled in this art 4 eluding at least onestage having an electron discharge device, a source of one predeterminedpotential connected to the grid of said electron discharge device, asecond source of another prede- 5 iter'mined potential connected to thecathode of I said electron discharge device, said potentials being sorelated that full gain throughsaid electron discharge device is normallyproduced, a source of high potential connected to the plate circuit ofsaid high frequency amplifier, a diode having a plate and a cathode, aresistor having a high value relatively to the impedance of the thatthere are modifications of the preferred enibodiment of this inventionas. described above, which may be made while" still embodying the broadnovel features of the same. For example, it is to be clearly understoodthat the voltages mentioned in describing the'operation of the inventionwere given merely by way of example,

and that these will vary in accordance with the constants of the circuitand the types of tubes used. While the. audio frequency amplifier hasbeen illustrated as comprising a plurality of stages, the invention willoperate with a-suitable amplifier havingone controlled stage.Furthermore, while the audio frequency amplifier tube to diode connectedintermediate said source of grid potential and the plate of said diode,means conto reduce the audio frequency amplifier gain.

be controlled has been illustrated as a pentode, I

the invention will operate properly with any suitable triode or tetrode.The rectifier illustrated as a diode it, can be replaced by any suitablerectifying mean which is capable of use with the potentials to be used.

Accordingly, while for the of illustration I have described oneembodiment of my invention, it is to be understood that I do not in-'tend suchinventlon to be limited thereto.

What I claim is:

1. In combination, a high frequency amplifier, a source of audiofrequency. an audio frequency amplifier connected thereto, means forcoupling said audiofrequency amplifier to the plate circult of said highfrequency amplifier to-modulate the output thereof, said audio frequencyamplifier including at least one stage having an electron dischargedevice, a source of one predetermined potential connected to a grid ofsaid electron discharge device, a second source of another predeterminedpotential cdnnected to the cathode of said electron discharge device,saidpotentials being so related that full gain through said electrondischarge device is normall produced, a source of high potentialconnected to the plate circuit of said high frequency amplifier, aresistor in the plate circuit of said high frequency amplifier, a diodehaving its plate connected to the grid of said electron dischargedevice, and

means for connecting the cathode of said diode across said resistor,whereby whenever the mean of said audio frequency modulating voltage andsaid high potential source falls below the potential on the plate ofsaid diode, the latter becomes conductive and reduces the potential Onthe grid of said electron discharge device to reduce the audio frequencyamplifier gain.

2. In combination, a high frequency amplifier, a source of audiofrequency, an audio frequency amplifier connected thereto, means forcoupling 3. In combination, a high frequency amplifier, a source ofaudio frequency, an audio frequency amplifier connected thereto, meansfor coupling said audio frequency amplifier to the platooncult of saidhigh frequenc amplifier to modulate the output thereof, said audiofrequency amplifier including at least one stage having an electrondischarge device, a source, of one predetermined potential connectedtothe grid of said electron discharge device, a second source of anotherpredetermined potential connected to the cathode of said electrondischarge device, said potentials being so related that full gainthrough said electron discharge device is normally produced, a source ofhigh potential connected to the plate circuit of said high frequencyamplifier, a diode having a plate and a cathode, a smoothing filternetwork intermediate the plate of said diode and the grid of saidelectron discharge device, means connectingthe cathode of said diode tothe plate circuit of said high frequency amplifier so as to beresponsive to the combined impressed audio frequency modulating voltageand said high potential source, and means for heating the cathode ofsaid diode with a portion of the plate current of said high frequencyamplifier, whereby, whenever, the mean of said audio frequencymodulating voltage and said high potential source falls below thepotential on the plate of said diode, the latter becomes conductive andreduces the potential on the grid of said electron discharge device toreduce th audio frequency amplifier gain. 4. The combination accordingto claim 1, in

combination with a resistor having a high-value relatively to theimpedance of the diode, connected intermediate the source of gridpotential and the plate of said diode.

5. In combination, a high frequency amplifier, a source of audiofrequency, an audio frequency amplifier connected thereto, means forcoupling said audio frequency amplifier to the plate circuit of saidhigh frequency amplifier to modulate the output thereof, said audiofrequency amplifier said audio frequency amplifier to the plate circuitof said high frequency amplifier to modulate the output thereof, saidaudio frequency amplifier inincluding at least one stage having anelectron discharge device, a source of one predetermined potentialconnected to the grid of said electron discharge device, a second sourceof another predetermined potential connected to the cathode of saidelectron discharge device, said potentials being so related that fullgain through said electron discharge device is normally produced, asource of high potential connected to the plate circuit of said highfrequency amplifier, a diode having a plate and a cathode, a resistorhaving a high value relatively to the impedance of the diode connectedintermediate said source of grid potential and the plate oi? said diode,a smoothing filter network intermediate the plate of said diode and thegrid or said electron discharge device, a m

